1. Field of the Invention
The present invention relates to a heat-curing organopolysiloxane composition, and in particular to an organopolysiloxane composition which cures into a very durable and very fatigue-resistant silicone rubber.
2. Background Information
Because silicone rubbers retain the properties of a rubbery elastomer over a range of temperatures, they are widely used as keyboards, valve packings, uniform-motion joint boots and electric wire coatings. However, compared with other organic rubbers, silicone rubbers generally have an inferior mechanical strength and, in particular, they are easily fatigued by the application of repeated deformation from the exterior.
Accordingly, in order to improve the aforementioned properties, silicone rubbers have been proposed in which organopolysiloxane with a low vinyl group content is blended with organopolysiloxane with a high vinyl group content. This improves the mechanical properties and particularly the tear strength. Also, silicone rubbers have been proposed in which organopolysiloxane with a low vinyl group content is blended with organopolysiloxane with a high vinyl group content and with organohydrogenpolysiloxane. This improves both the heat resistance and tear strength.
U.S. Pat. No. 3,652,475, issued Mar. 28, 1972, describes a heat curable elastomeric silicone composition which cures to yield a silicone rubber having a high tear strength. The composition consists of polydiorganosiloxane having a degree of polymerization of at least 3,000 and containing at most 0.3 mole percent of vinyl group containing siloxane units, polydiorganosiloxane having a degree of polymerization of at least 3,000 and containing from 5 to 20 mole percent of vinyl group containing siloxane units polydiorganosiloxane having a degree of polymerization of from 10 to 1,000 and containing from 5 to 90 mole percent vinyl group containing siloxane units, silica filler, and organic peroxide.
U.S. Pat. No. 3,671,480, issued June 20, 1972, describes a heat curable elastomeric silicone composition comprising a mixture of two polydiorganosiloxanes, each of which contains vinyl unsaturation, a silica filler, a polydiorganohydrogensiloxane and a platinum compound.
U.S. Pat. No. 3,660,345. issued May 2, 1972, describes organopolysiloxanes which are convertible to elastomers having high tear strength and resiliency. The compositions comprise either a two component blend of a vinyl containing organopolysiloxane gum having minor amounts of chemically combined siloxy units with at least one vinyl radical attached to silicon, and a vinylpolysiloxane having up to major amounts of chemically combined siloxy units with at least one vinyl radical attached to silicon, or a three component blend comprising an organopolysiloxane gum free of chemically combined siloxy units with at least one vinyl radical attached to silicon in combination with the aforementioned two component blend.
U.S. Pat. No. 4,539,357, issued Sept. 3, 1985, relates to a silicone elastomer having a high tear strength comprising a blend of vinyl-containing gums, a silica reinforcing filler, a hydride crosslinking agent, and a peroxide curing catalyst. One of ordinary skill in this art would recognize that the hydride crosslinking agent of this patent (Bobear '357) corresponds to the organohydrogen polysiloxane component (c) of the present invention. Bobear '357 teaches that the organohydrogen polysiloxane typically has a viscosity ranging from 10 to 1000 centipoises at 25.degree. C.
However, while the aforementioned methods can provide silicone rubbers in which the mechanical property of tear strength is improved, they are not necessarily satisfactory with regard to the production of highly durable silicone rubbers with an excellent fatigue resistance, i.e., bending fatigue resistance and elongation fatigue resistance.
Various methods were examined by the present inventors in order to resolve the aforementioned problems of the prior art, and it was discovered that a silicone rubber, in which there is a localization in the distributed density of the crosslinking bonds of the organopolysiloxane gum forming the skeleton of the silicone rubber and which also is reinforced with a specific organopolysiloxane and a specific silica, has an excellent fatigue resistance as well as a high durability. The invention was developed based on this observation.